End face mechanical seals, also referred to as mechanical face seals or simply as mechanical seals, are commonly used to isolate fluids in rotating equipment, such as pumps, mixers, blowers, and compressors. Typically, a pressure differential exists across the seal faces of a mechanical seal, which may vary in intensity during operation, but which typically remains constant in direction.
Mechanical seals generally require that a lubricant or other “cushioning substance” occupy the gap between the stationary and rotating seal face, so as to minimize frictional heating and premature wear. For many applications, the process fluid is a liquid that is used as the seal lubricant, whereby it is allowed to enter the seal and slowly leak past the seal faces. In other applications, for example when the process fluid is non-lubricating and/or toxic, a separate, pressurized lubrication system is provided that introduces a liquid lubricant into the seal, which then slowly leaks into the process, thereby preventing any escape of the process fluid into the surrounding environment.
In certain applications, the seal faces of a mechanical seal cannot be lubricated by a liquid. An example is a seal that separates a compressor from a bearing housing in a turbocharger of an internal combustion engine.
Turbochargers are used for both commercial and racing applications in a wide variety of vehicle engines, including gasoline and diesel engines. They pose some unique and challenging operating conditions for a seal. Temperatures in a turbocharger can rise up to 200° C., and shaft speeds can reach 120,000 rpm. In addition, the pressure within the compressor can range from vacuum up to more than 5 bar, which means that the pressure differential across the seal can change directions.
Accordingly, a seal for a turbocharger must be able to minimize the loss of compressed air from the compressor side of a seal into the bearing housing when the pressure in the compressor is high, for example during normal turbocharger operation and when the engine and turbocharger are rapidly accelerating. The seal must also be able to minimize leakage of oil from the bearing housing side of the seal into the compressor side during rapid engine deceleration, when vehicle braking occurs and a vacuum is created on the compressor side of the seal that tries to pull oil from the bearing housing side of the seal into the compressor.
Poor performance of the seal in a turbocharger therefore can cause both loss of compressed air and leakage of oil into the compressor, thereby reducing the performance of the turbocharger, and hence reducing the overall performance of the engine. Over the years, several approaches have been proposed for providing a seal between the bearing housing and compressor sections of a turbocharger. These have ranged from piston rings to labyrinth seals.
Mechanical seals designed for gas applications would be an attractive solution for turbochargers. While they tend to be somewhat complex, they offer distinct advantages in providing low air and/or oil leakage, as compared to the other types of seals that are normally used. One such mechanical seal design for a turbocharger is disclosed in U.S. Pat. No. 6,325,380 to Feigl, et al. Feigl discloses the use of a non-contacting mechanical gas seal that includes specific hydrodynamic features on at least one of the seal faces. In this design, illustrated in FIGS. 1 and 2, the air-filled compressor side is located at “A” and the bearing housing filled with oil is located at “B.” The seal faces are items 3 (stationary face) and 5 (rotating face).
FIG. 2 illustrates the use by Feigl of micro machined hydrodynamic face features 14 located at the inner diameter of the stationary face 3. These features 14 can also be located on the rotating face 5 as well. Feigl's hydrodynamic seal face features 14 are micro machined grooves 14 in the seal face 3. Under rotating conditions, these grooves 14 pump and compress air into the gap between the faces 3, 5 and thereby generate lift between the faces 3, 5 for non-contact operation. The amount of lift (or face separation) can be controlled by careful design of the hydrodynamic features 14, based on the operating conditions of the turbocharger, especially its operational speed and the pressure. All else being equal, the separation between the faces 3, 5 will increase with increasing speed and pressure. Likewise, when the speed and pressure are decreased, the face separation will decrease as well.
One of the challenges with the Feigl approach, however, is that the hydrodynamic face features do not function when rapid car braking causes the turbocharger to reduce in speed, a vacuum is created within the compressor, and the pressure differential across the seal changes direction. Under these operating conditions, the face separation is dramatically reduced, because the hydrodynamic face features require air and compression to function appropriately. As a result, rapid deceleration can lead to unacceptable contact of the seal faces 3, 5 in the Feigl design, and can result in undue wear and seal face damage.
What is needed, therefore, is a mechanical seal designed for gas applications that can minimize leakage in both directions across the seal, so that undue leakage and wear of the seal faces is avoided even when the pressure differential across the seal changes direction, such as during rapid deceleration of a turbocharger.